Being the new guy here with my most recent topics I have been advised to get a heater, I was wondering as to which ones are best? I would like to have one with a built in timer but I am completely new to the fish keeping world and what not so I don't know much about heaters in general. I was sure that everyone here would have a MUCH MUCH MUCH better idea on the correct one!

It is a 20 gallon tank (the old ones, like from the 90s) I don't think it's high but I do know for a fact that its not long. It will be a tropical community tank with things along the lines of Zebras, maybe a male betta, eventually a pleco of some sort and some black neon tetras. I have a few live plants if that makes a difference as well.

I will quit referring to myself as the "new guy" soon I promise Lol

If possible I would like to keep the cost of this heater to about $20.00 or under!

The Aqueon Pro heaters are very nice although a bit costly. They don't have a timer but are very accurate and aren't an eye-sore in the tank.
You could always get a cheap light timer and plug the heater into that.

A more economical one are the Hydor Theo heaters, which might not be as good quality but from reviews I have read they do the job. For your sized tank you would probably want a 100 watt size.

Heaters don't have timers but do have thermostats so the temperature reaches a set point, heater turns off until the water fall below a set point and heater comes on again. You just leave them on all the time.
I can't help with specific models as you use a different voltage but get one that is fully submersible and 100 watts. Tetra probably make one within your budget.